1. Field of the Invention
The present invention relates to an automatic transmission to be used in a vehicle such as an automobile and, more particularly, to an automatic transmission for setting a plurality of gear stages by frictional engagement means such as clutches and brakes.
2. Discussion of the Related Art
An automatic transmission to be mounted on a vehicle such as an automobile is desired to have a small size and a light weight, to facilitate control of its shifts, to have low shifting shocks and to be able to set a number of gear stages at proper gear ratios. In order to increase the number of gear stages to be set, it is sufficient to increase the number of planetary gear sets composing the gear train or the increase the number of frictional engagement means such as clutches or brakes so that the connections and stationary states of the rotary components of the gear train may be changed in various manners. We have already proposed an automatic transmission which is enabled to have major gear stages of forward 1st to 5th speeds having gear ratios of geometric series and reverse one speed thereby to set intermediate stages between the 2nd and 3rd speeds and between the 3rd and 4th speeds by composing the gear train majorly of three planetary gear sets. In the automatic transmission thus proposed, the connections and stationary states of the components of the planetary gear sets composing the gear train can be diversified so that the number of gear stages to be set can be increased while providing a plurality of kinds of combination patterns of engagement/release (i.e., engagememt/release patterns) of the frictional engagement means for setting a predetermined one of gear stages. In some engagement/release pattern, the rotary components may have their numbers of revolution varied or unvaried. As a result, the shifting shocks, the shift controllability, and the durability of the frictional engagement means are seriously influenced by the manner how to select the frictional engagement means for setting a predetermined gear stage. Therefore, we have already proposed a shift control method for an automatic transmission capable of selecting a plurality of engagement/release patterns for setting a predetermined gear stage. By this method, the gear stage is set by selecting such one of plural kinds of engagement/release patterns as to reduce load torque to be applied to the frictional engagement means, as disclosed in Japanese Patent Application No. Hei 1-267257.
The load torque to be applied to the frictional engagement means for setting a predetermined gear stage by changing the transmission passages of the drive force in the gear train composed of a plurality of planetary gear sets can be determined as a ratio to input torque inputted to the gear train, on the basis of the gear ratio (i.e., the ratio between the tooth number of a sun gear and the tooth number of a ring gear) of each planetary gear set. In our above-specified method, the gear stage is set by selecting such an engagement/release pattern as to reduce that ratio. In case a shift is to be carried out, on the other hand, the shift controllability has to be improved with less shifting shocks by operating a oneway clutch to execute the shift or by operating the frictional engagement means equipped with an accumulator.
Incidentally, the load torque to be applied to each frictional engagement means can be grasped as the ratio to the input torque, as described above. Since, however, the actual load torque is varied with the input torque, it may raise no problem in case the input torque is low, even if the ratio to the input torque is great.
Even in case the input torque is low, it is conceivable to execute the aforementioned method according to our proposal so that the engagement/release pattern is selected by considering the shift controllability. Then, no serious problem will arise for practical purposes, but an unnecessary control is performed by changing the frictional engagement means after or before the shift, so that the controls for the shift are disadvantageously complicated.
If, on the other hand, the engagement/release pattern for reducing the load torque is selected, it is possible to reduce the size of the frictional engagement means and to improve their durability. However, the transmission torque capacity of the frictional engagement means is determined by the oil pressure supplied, and the control oil pressure is reduced in case the ratio of the load torque is low. As a result, there arises another problem that the necessary control oil pressure is too low to control or that the control accuracy deteriorates, in case the input torque is low. Thus, the method of determining the engagement/release pattern with reference to the load torque cannot be applied to all the situations.